A successor communication scheme to W-CDMA (Wideband Code Division Multiple Access) and HSPA (High Speed Packet Access), that is, Evolved UTRA and UTRAN (also referred to as LTE (Long Term Evolution) or Super 3G) is being discussed in W-CDMA standardization group 3GPP (3rd Generation Partnership Project). In the E-UTRA, for example, OFDMA (Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single-Carrier Frequency Division Multiple Access) are utilized for downlink and uplink, respectively. (See non-patent document 1, for example.)
The OFDMA is a transmission scheme where a frequency band is divided into multiple smaller frequency bands (subcarriers) and data is transmitted in the individual subcarriers. The OFDMA achieves fast transmission by arranging the subcarriers in the frequency band densely while partially overlapping the subcarriers, resulting in higher frequency utilization efficiency.
The SC-FDMA is a transmission scheme where a frequency band is divided and the different frequency bands are utilized for multiple terminals for reduction in interference among the terminals. Since the SC-FDMA has characteristics of reduced variations of transmit power, it can reduce power consumption in the terminals and realize broader coverage.
In the LTE, two types of CPs (Cyclic Prefixes) having different lengths, a long CP and a short CP, are provided for reducing influence of inter-symbol interference due to delayed waves. For example, the long CP may be for a cell having a large cell radius and be applied in transmission of MBMS (Multimedia Broadcast Multicast Service) signals, and the short CP is applied for a cell having a smaller cell radius. In the long CP applied case, six OFDM symbols are provided in one slot, and in the short CP applied case, seven OFDM symbols are provided in one slot.
Meanwhile, in a radio communication system using the W-CDMA, the LTE and so on, a mobile station must generally detect a cell having higher radio quality for itself based on a synchronization signal or others at power up, in standby period, during communication, at discontinuous reception in communication or at other timings. This process is called cell search in a sense that the mobile station searches for the cell to be connected to establish a radio link. In general, a cell search method is selected based on an amount of time required for the cell search and an amount of load of the mobile station for the cell search. In other words, the cell search method may be selected to achieve the cell search in a shorter time period at a smaller amount of processing load.
In the W-CDMA, the cell search uses two types of synchronization signals, a primary SCH (P-SCH) and a secondary SCH (S-SCH). Also in the LTE, the two types of synchronization signals P-SCH and S-SCH are used in the cell search.
In one cell search method, for example, the P-SCH having one sequence and the S-SCH having multiple sequences may be transmitted every 5 ms (non-patent document 2). In this cell search method, downlink reception timings from cells are identified based on the P-SCH, and frame reception timing and cell specific information such as a cell ID or a cell group ID are identified based on the S-SCH. Here, a channel estimate derived from the P-SCH can be generally used in demodulation and decoding of the S-SCH. Also, if the cell IDs are grouped, a relevant cell ID is selected among the cell IDs belonging to a detected cell group ID. For example, the cell ID may be determined based on a signal pattern of a pilot signal. In another example, the cell ID may be determined based on demodulation and decoding of the P-SCH. Alternatively, the cell ID may be included as an information element of the S-SCH without grouping the cell IDs. In this case, a mobile station could detect the cell ID at the S-SCH demodulation and decoding timing.
In the case where the above-mentioned cell search method is applied, however, the S-SCH transmitted from multiple cells in different sequences would be demodulated and decoded based on the channel estimate derived from the P-SCH transmitted from the multiple cells in the same sequence in an inter-station synchronization system where signals from the individual cells are synchronized. For this reason, there is a risk that transmission characteristics of the S-SCH may be degraded. Here, the transmission characteristics include time required for the cell search. On the other hand, in an inter-station asynchronization system where signals from individual cells are not synchronized, reception timings of the P-SCH sequences transmitted from the multiple cells are different from each other, which may not cause the above-mentioned problem.
In order to prevent the degradation of the S-SCH characteristics in the inter-station synchronization system as stated above, a cell search method using two or more P-SCH sequences, such as a cell search method using three P-SCH sequences, is being discussed (non-patent document 2). Alternatively, in order to prevent the degradation of the S-SCH characteristics in the inter-station synchronization system as stated above, a method for transmitting the P-SCH at different transmission intervals for different cells is proposed (non-patent document 3). According to these methods, the P-SCH having different reception timings from the multiple cells can be used in demodulation and decoding of the S-SCH, which can prevent the degradation of the S-SCH characteristics as stated above.
Meanwhile, from the viewpoint of cell design, it may be preferable that a greater number of P-SCH sequences in non-patent document 2 and more kinds of transmission intervals in non-patent document 3 be used. This is why there is a higher likelihood that the same P-SCH sequence or the same P-SCH transmission interval is used for adjacent cells in the cases of a smaller number of P-SCH sequence and less kinds of P-SCH transmission intervals, which would increase a likelihood that the S-SCH characteristics may be degraded in the inter-station synchronization system.
Also, there is trade-off between the amount of time required for the cell search, that is, transmission characteristics of the cell search, and the processing load of a mobile station for the cell search. Thus, it is desirable that it can be selected through parameter settings or operation methods on which of the transmission characteristics of the cell search or the processing load of the mobile station for the cell search greater importance is to be put.